This invention relates to steel core serving as reinforcing materials of elastomeric articles of tires, belts or the like.
Steel cores are generally used as reinforcing materials for rubber articles, which include tires of motorcars, monorails or building vehicles, conveyor belts, hoses, etc.
Nowadays, the motorcars' tires, for example, are required to have high performance and flatness, and to be lightened in weight and lowered in cost. For satisfying these requirements, it is necessary that not only the rubber itself as a matrix be of excellent quality, but also the steel core itself to be embedded in the matrix has stable structure.
The steel core is, as known, formed by combining a plurality of very fine steel wires. The existing steel cores are structured with a plurality of filaments twisted together at a certain pitch. Therefore an outer contour in a cross section transverse with an axial line has a rounded shape (FIG. 8-A) or a polygonal shape nearly round (FIG. 8-B or FIG. 8-C).
Due to such a structure, elastic rigidities of the steel cores are equal in X-direction and Y-direction. Accordingly, an elastomeric article embedded with the steel cores, e.g., the belt for tire has an equal elastic rigidity vertically (in thickness) and laterally (in width). So, such a quality of the article does not meet the movement performance of the tire satisfactorily. As well, the belt is repeatedly given the bending stresses during a long period of life of a radical tire. Since the belt is poor in vertical flexibility, it has trouble countering against fatigue-weakening.
With respect to lightening weight of the tire and the belt conveyer, it is effective to decrease the number of the steel cores to be buried in the belt. However, if the number of buried steel cores per unit area of the belt were decreased, the rigidity of the belt would be lowered, as would the resistance to nails, rocks and so on. Therefore, a satisfactory lightening in weight could not be achieved.
With respect to lowered cost, it is, as known, effective to make the thickness of the gauge of a calendar sheet composing the belt thin. But since the conventional steel core has its cross section perpendicular to the axial direction, which has an equal dimension in the vertical and lateral directions, the thickness of the gauge could not be thinned. For accomplishing the object, the filament should be made thin. However, this work involves substantial difficulties and involves the higher cost.
U.S. Pat. No. 4,464,892 (Jacob Kleijwegt) or No. 4,545,190 (Grover W. Rye) propose the steel cores. The former teaches that a strand is formed by twisting together two filaments and helically disposing therearound a single filament of the same thickness as said filament. The latter teaches that helixes formed by a plurality of filaments have a pitch length of 5 to 30 mm, and the pitch length of the helixes of the plurality of filaments is equal to the lay length of the single filament twisted with the plurality of filaments, and said filament is twisted with said strand with a lay length that is equal to said pitch length.
These conventional techniques provide twisting or helical shape to the strands, so that the cross sectional area transverse with the axial line of the core is changed at particular locations. Such requirements as flexibility, faculty of bending fatigue, flatness or weight lightening could not be satisfied.